Low T4 and low FT4I in seriously ill patients: concise communication

Of 128 euthyroid hospitalized patients with nonthyroidal illnesses (NTI), 33% had a low total thyroxine (T4). Forty-three percent of the latter patients had a low free thyroxine index. In euthyroid patients with NTI, free T4 was, with rare exceptions, within the range for well euthyroid controls. The free T4, as determined by two different radioimmunoassays, was diagnostically low in 34 of 37 hypothyroid patients without NTI. However, in hypothyroid patients with NTI, the free T4 determination was less informative. Only four of ten patients had a low free T4 by the Corning test, and none of ten were low by the Clinical Assays test. Our data suggest that patients with NTI frequently have low T4 and low FT4I, despite being euthyroid. Low free T4 strongly suggests hypothyroidism, but normal free T4 in patients with NTI does not exclude hypothyroidism.